1. Field of the Invention
This invention relates to improvements in an oil pump for supplying oil, and more particularly to an oil pump for supplying lubricating oil to necessary parts in an automotive engine.
2. Description of the Prior Art
A variety of oil pumps have been proposed and put into practical use. A typical one of them is disclosed in Japanese Utility Model Publication No. 1-76582, in which inner and outer cylindrical rotors are rotatably disposed inside a pump casing. The inner rotor is fixedly mounted on one end section of a drive shaft and formed with an external gear which is in mesh with an internal gear of the outer rotor. The drive shaft is formed at the peripheral surface of its one end section with two flat surface portions which are parallel with and spaced from each other. During operation of the oil pump, the torque from the drive shaft is transmitted through the two flat surface portions to the inner rotor.
Additionally, Japanese Utility Model Provisional Publication No. 63-126506 discloses an oil pump similar to the above-discussed one. In this oil pump, an annular drive spacer is interposed between a drive shaft and an inner rotor. The drive spacer is formed with the two parallel flat surface portions and fixed to the drive shaft by means of a key.
However, drawbacks have been encountered in all of the above-discussed conventional oil pumps. That is to say, since driving of the inner rotor is carried out through the two parallel flat surface portions, the inner rotor is unavoidably pushed and moved toward an oil inlet chamber side under oil discharge pressure at an oil outlet chamber side when the two parallel flat surface portions come to a position to extend in the direction connecting the oil inlet and outlet chambers. As a result, the external gear of the inner rotor and the internal gear of the outer rotor interfere with each other thereby generating gear noise.